Cosmic Rays and the Development of Particle Physics
3.4 The Recent Years
3.4 The Recent Years
Things went more or less as predicted by Leprince-Ringuet.
Between the 1950s and the 1990s most of the progress in fundamental physics was due to accelerating machines. Still, however, important experiments studying cosmic rays were alive and were an important source of knowledge.
Cosmic rays are today central in the field of astroparticle physics, which has grown considerably in the last 20 years. Many large projects are active, with many different goals, including, for example, the search for dark matter in the Universe.
Gamma-ray space telescopes on satellites like theF er mi Large Area Telescope (F er mi-LAT) and AGILE, and the PAMELA and AMS-02 magnetic spectrometers, provided cutting-edge results; PAMELA in particular observed a yet unexplained anomalous yield of cosmic positrons, with a ratio between positrons and electrons growing with energy, which might point to new physics, in particular related to dark matter. The result was confirmed and extended to higher energies and with unprecedented accuracy by the AMS-02 detector onboard the International Space Station.
The study of very highest energy cosmic ray showers, a century after the discovery of air showers by Rossi and Auger, is providing fundamental knowledge on the spectrum and sources of cosmic rays. In particular the region near the GZK cutoff is explored. The present-day largest detector, the Pierre Auger Observatory, covers a surface of about 3000 km2in Argentina.
The ground-based very high-energy gamma telescopes HAWC, H.E.S.S., MAGIC, and VERITAS are mapping the cosmic sources of gamma rays in the TeV and multi- TeV region. Together with theF er mi satellite, they are providing indications of a link between the photon accelerators and the cosmic ray accelerators in the Milky Way, in particular supernova remnants. Studying the propagation of very energetic photons traveling through cosmological distances, they are also sensitive to possible violations of the Lorentz invariance at very high energy, and to photon interactions with the quantum vacuum, which in turn are sensitive to the existence of yet unknown fields. A new detector, CTA, is planned and will outperform the present detectors by at least an order of magnitude.
The field of study of cosmic neutrinos registered impressive results. In the anal- ysis of the fluxes of solar neutrinos and then of atmospheric neutrinos, studies per- formed using large neutrino detectors in Japan, US, Canada, China, and Italy have demonstrated that neutrinos can oscillate between different flavors; this phenomenon requires that neutrinos have nonzero mass—present indications favor masses of the order of tenths of meV. Recently the IceCube South Pole Neutrino Observatory, a km3 detector buried in the ice of Antarctica, has discovered the first solid evidence for astrophysical neutrinos from cosmic accelerators (some with energies greater than 1 PeV). With IceCube, some ten astrophysical neutrinos per year (with a∼20%
background) have been detected in the last 5 years; they do not appear within the present statistics to cluster around a particular astrophysical source.
Finally, a handful of gravitational wave events have been detected in very recent years. In 2015, the LIGO/Virgo project directly detected gravitational waves using laser interferometers. The LIGO detectors observed gravitational waves from the merger of two stellar-mass black holes, matching predictions of general relativity.
These observations demonstrated the existence of binary stellar-mass black hole systems and were the first direct detection of gravitational waves and the first obser- vation of a binary black hole merger. Together with the detection of astrophysical neutrinos, the observations of gravitational waves paved the way for multimessen- ger astrophysics: combining the information obtained from the detection of photons, neutrinos, charged particles, and gravitational waves can shed light on completely new phenomena and objects.
Cosmic rays and cosmological sources are thus again in the focus of very high- energy particle and gravitational physics. This will be discussed in greater detail in Chap.10.
Further Reading
[F3.1] P. Carlson, A. de Angelis, “Nationalism and internationalism in science: the case of the discovery of cosmic rays”, The European Physical Journal H 35 (2010) 309.
[F3.2] A. de Angelis, “Atmospheric ionization and cosmic rays: studies and mea- surements before 1912”, Astroparticle Physics 53 (2014) 19.
[F3.3] D.H. Griffiths, “Introduction to Quantum Mechanics, 2nd edition,” Addison- Wesley, Reading, MA, 2004.
[F3.4] J. Björken and S. Drell, “Relativistic Quantum Fields,” McGraw-Hill, New York, 1969.
Exercises
1. The measurement by Hess.Discuss why radioactivity decreases with elevation up to some 1000 m and then increases. Can you make a model? This was the subject of the thesis by Schrödinger in Wien in the beginning of twentieth century.
2. Klein–Gordon equation.Show that in the nonrelativistic limitE mc2the posi- tive energy solutionsΨ of the Klein–Gordon equation can be written in the form
Ψ (r,t) Φ(r,t)e−mc
2 t,
whereΦsatisfies the Schrödinger equation.
3. Antimatter.The total number of nucleons minus the total number of antinucleons is believed to be constant in a reaction—you can create nucleon–antinucleon pairs. What is the minimum energy of a proton hitting a proton at rest to generate an antiproton?
4. Fermi maximum accelerator.According to Enrico Fermi, the ultimate human accelerator, the “Globatron,” would be built around 1994 encircling the entire Earth and attaining an energy of around 5000 TeV (with an estimated cost of 170 million US dollars at 1954 prices.). Discuss the parameters of such an accelerator.
Exercises 107
5. Cosmic pions and muons.Pions and muons are produced in the high atmosphere, at a height of some 10 km above sea level, as a result of hadronic interactions from the collisions of cosmic rays with atmospheric nuclei. Compute the energy at which charged pions and muons, respectively, must be produced to reach on average the Earth’s surface.
You can find the masses of the lifetimes of pions and muons in Appendix D or in your Particle Data Booklet.
6. Very high-energy cosmic rays.Justify the sentence “About once per minute, a single subatomic particle enters the Earth’s atmosphere with an energy larger than 10 J” in Chap.1.
7. Very-high-energy neutrinos.The IceCube experiment in the South Pole can detect neutrinos crossing the Earth from the North Pole. If the cross section for neutrino interaction on a nucleon is(6.7×10−39E)cm2withEexpressed in GeV (note the linear increase with the neutrino energyE), what is the energy at which half of the neutrinos interact before reaching the detector? Comment on the result.
8. If aπ0from a cosmic shower has an energy of 2 GeV:
(a) Assuming the twoγrays coming from its decay are emitted in the direction of the pion’s velocity, how much energy does each have?
(b) What are their wavelengths and frequencies?
(c) How far will the average neutral pion travel, in the laboratory frame, from its creation to its decay? Comment on the difficulty to measure the pion lifetime.